Anisometric Molecules Research Articles

Formation of a columnar liquid crystal in a simple one-component system of particles.

We report a molecular dynamics simulation demonstrating that a columnar liquid crystal, commonly formed by disc-shaped molecules, can be formed by identical particles interacting via a spherically symmetric potential. Upon isochoric cooling from a low-density isotropic liquid state the simulated system underwent a weak first order phase transition which produced a liquid crystal phase composed of parallel particle columns arranged in a hexagonal pattern in the plane perpendicular to the column axis. The particles within columns formed a liquid structure and demonstrated a significant intracolumn diffusion. Further cooling resulted in another first-order transition whereby the column structure became periodically ordered in three dimensions transforming the liquid-crystal phase into a crystal. This result is the first observation of a columnar liquid crystal formation in a simple one-component system of particles. Its conceptual significance is in that it demonstrated that liquid crystals that have so far only been produced in systems of anisometric molecules can also be formed by mesoscopic soft-matter and colloidal systems of spherical particles with appropriately tuned interatomic potential.

A molecular based continuum approach for the dielectric permittivity of liquids and liquid crystals

A method for the calculation of the dielectric permittivity of isotropic and anisotropic homogeneous fluids is presented which, in the framework of the continuum approximation, adopts a realistic description of the molecular features, so overcoming some of the limits of the Onsager model. The Poisson equation for the molecular charge distribution contained in a cavity in a dielectric continuum in the presence of an external field is solved by a boundary element technique, which allows a detailed description of the cavity shape associated with a given molecular structure. The charge distribution is described in terms of point charges derived from ab initio calculations in vacuum, in addition to a set of interacting atom dipoles induced by all the electric fields experienced by the molecule in the condensed phase. The link between molecular features and bulk properties is established in a general way suitable for isotropic liquids and nematic phases, through the orientational distribution function of the molecule interacting with the applied field and the surrounding fluid. Numerical results are reported for the liquid phase of a set of selected organic compounds of different shape and polarity, and for the isotropic and nematic phases of 4-n-pentyl-4′-cyanobiphenyl (5CB). They show that a realistic description of the molecular features can have dramatic effects in the case of strongly anisometric molecules.

Molecular organization in liquids consisting of anisometric molecules: Monte Carlo simulations and comparison with theories

Monte Carlo computer simulations of systems of rigid anisometric molecules with axial ratio x 3, 4 and 5 have been performed at densities comparable to those of the mesogenic groups in liquid crystalline materials. The agreement between computer experiments and Flory-Ronca-Irvine theory is quite good. In particular, the nematic-isotropic transition temperatures found in the simulations are within 20 K of those predicted by the theory in all cases examined with x 3 and 4, and only slightly higher when x 5. Furthermore, the orientational distributions of the molecular long axes with respect to the nematic director are always nearly coincident with those evaluated according to the FRI theory, provided that the order parameter is the same. However, the order parameters at the transition points are smaller than expected on the basis of the theory, and closer to experimental values. Calculations performed by changing the size of the simulated systems prove that the reported results are size independent, while size dependent results are obtained with smaller systems.

The effect of molecular shape anisotropy on surface melting

We report molecular dynamics (MD) simulations of the behaviour of the (101̄0) and (0001) crystal-vapour interfaces of Lennard-Jones dimers near the triple point. For slightly anisometric molecules, where the bulk melts from an orientationally disordered state, both interfaces were found to surface melt, with behaviour resembling that of the atomic Lennard-Jones system. In contrast, at larger anisometries, where the molecules are orientationally ordered close to the triple point, the (101̄0) face exhibits a first-order surface melting transition, while the (0001) face does not surface melt at all.

Diffusion of plasticizers in rubbery poly(vinyl chloride): effects of molecular shape

Self-diffusion of commercial plasticizers in PVC was studied by NMR at temperatures between 75 and 175°C and at concentrations c from 10 to 60 wt % plasticizer. Diffusion of the neat plasticizers in CCl 4 was also measured. The temperature dependance of the diffusion coefficient D in PVC generally followed the WLF expression. Plasticizer molecular shape affected D in PVC but not in liquid solution. It is concluded that the motion of these highly anisometric molecules is itself anisotropic provided the host molecules offer effective lateral constraints, which need not arise from cross-links or entanglements

The intrinsic polarizability tensor of non-polar anisometric molecules from a combination of measurements in a condensed phase

Rigid non-polar molecules whose shape is convex and can be approximated by an ellipsoid arc considered. Using the formalism of the "effective polarizability increments" previously developed, it is shown that, starting from combination of different experiments bearing on a solution of such molecules in an isotropic solvent, it is possible to determine the principal elements of the intrinsie (in vacuo) polarizability tensor of the molecule under study. These experiments are the dielectric increment of the solution, its Kerr constant, and the NMR splitting under an electric field of certain atoms of the solute.

Anisometric molecules in dense fluids

The orientational contributions to the Kerr constant (electric birefringence) in dilute solutions are calculated. These are the alignment of the polarizability tensors of the solutes (calculated in a previous article in this series), and the orientation of their permanent moments, which is also considered here. The model is that where each anisometric molecule is considered as embedded in an ellipsoïdal cavity digged in an isotropic continuum which represents the non-birefringent solvent. The final formula which is obtained is identical to that for dilute gases, except that the polarizability tensors in vacuo must be replaced by ‘effective incremental polarizability tensors’ α* previously defined in this Series, and that the permanent dipole components must be multiplied by corresponding elements of the tensorial internal field factor.

Anisometric molecules in dense fluids

On considère le modèle dans lequel une molécule polaire et ayant une polarisabilité tensorielle est logée dans une cavité ellipsoïdale ménagée dans un continuum diélectrique, ce dernier étant supposé répondre linéairement aux champs électriques. On calcule, comme dans l'article précédent [7], en présence d'un champ électrique appliqué, le dipole source du champ dipolaire moyen produit par la substitution de la cavité et de sa molécule au volume correspondant du continuum. Ici ce dipole source est calculé jusqu'à l'ordre 3 du champ appliqué. Il permet le calcul de la susceptibilité non linéaire x (3). On trouve pour celle-ci une expression formellement simple contenant un terme de saturation dipolaire, un terme d'alignement de la polarisabilité maximale, et un terme mélangeant ces deux effets. Ceci est analogue au résultat classique pour les gaz, mais le dipole permanent doit y être multiplié par le tenseur facteur de champ interne f, tandis que les polarisabilités principales in vacuo doivent y être remplacées par des ‘polarisabilités incrémentielles efficaces’ α jj *, identiques à celles définies dans les articles précédents. Chacune de celles-ci contient un terme anisotrope proportionnel à la polarisabilité in vacuo correspondante, mais aussi un terme négatif, également anisotrope, proportionnel au volume de la cavité. On montre que la théorie s'applique avec succès sur un exemple tiré de la littérature: le chloroforme dans le cyclohexane, ce dernier pouvant être considéré comme un solvant à comportement isotrope et linéaire.

Analysis of the depolarized Rayleigh light scattering spectrum of liquids composed of anisometric molecules: Nitrobenzene

Analysis of the central component of the depolarized Rayleigh light scattering spectrum of nitrobenzene is complex. Existing experimental data for nitrobenzene, including the concentration and temperature dependence of the Rayleigh linewidth, the concentration dependence of the depolarized Rayleigh intensity, and the translational-rotational coupling parameter, are analyzed using asymmetric top equations to obtain the orientational pair correlation parameters. The results of this analysis and the translational-rotational coupling parameter are used to explain the temperature dependence of the δ parameter introduced by Lempert and Wang [J. Chem. Phys. 72, 3490 (1980)]. Finally, an equation for the depolarized Rayleigh doublet for a liquid composed of asymmetric top molecules (nitrobenzene) is presented. This result is similar to previous results for liquids composed of symmetric top molecules.

Diffusion of organic vapors at low concentrations in glassy PVC, polystyrene, and PMMA

Diffusion coefficients of various C 1 to C 6 organic vapors, at concentrations 0.5 wt. percent, have been determined by gravimetric sorption rate measurements on emulsion and suspension-polymerized powder samples of PVC, polystyrene, and PMMA. Fickian diffusion kinetics were observed at the lowest concentrations, with a second-stage, relaxation-controlled sorption appearing at higher concentrations. In conjunction with published data for diffusivities of fixed gases in these polymers, the results indicate that diffusivity decreases exponentially, and that diffusion activation energy ( E D) increases linearly, with increasing diameter of “spherical” penetrant molecules (e.g., the noble gases, CH 4, SF 6, CCl 4, and neopentane). Much of the observed scatter in these correlations is attributable to uncertainty in the molecular diameters. For C 4 and larger n-alkanes and other elongated or flattened molecules, diffusivities are higher, and E D lower, than for spherical molecules of similar molar volume. This finding suggests that anisometric molecules are oriented and move along their long axes during diffusion through the glassy polymer matrix. Correlations of diffusivities with molecular dimensions suggests that transport of anisometric molecules is governed by a diameter smaller than the mean (equivalent sphere) diameter but larger than the minimum dimension of their extended-chain conformation. Among the three polymers studied, diffusivity of each penetrant, at a given temperature, decreases in the order polystyrene > PVC ≥ PMMA.

Anisometric molecules in dense fluids

Anisometric molecules in dense fluids

Anisometric molecules in dense fluids

On considere d'abord, dans le cadre d'un modele a cavite ellipsoidale, l'equation dielectrique de molecules anisometriques dans un solvant isotrope, en utilisant le champ interne de Onsager-Scholte...

Anisometric molecules in dense fluids

Anisometric molecules in dense fluids

Anisometric molecules in dense fluids

The intensity of depolarized light scattered by a solution of anisotropic molecules in a non-depolarizing solvent is considered within the framework of the ellipsoidal cavity model. It is shown that the depolarized intensity is given by the same formula as in dilute gases, but replacing the vacuum molecular polarizabilities α zz , etc., by effective polarizabilities α zz * defined as Here ε is the optical dielectric constant of the solution, a, b and c are the principal semi-axes of the ellipsoidal cavity, Sz is the shape-dependent depolarizing factor, and fz is the ε-dependent Onsager-Scholte internal field factor. This theory withstands the essential self-consistency test that the depolarization must be identically zero if the solute molecule is replaced by an isotropic ellipsoidal body of dielectric constant ε filling the cavity.

Coupling of velocity gradients to orientation densities

Velocity gradients and orientation densities are coupled in liquids composed of anisometric molecules, as is evidenced by flow birefringence and by the Rytov dip observed in VH depolarized light scattering. The coupling strength can be related to a dimensionless parameter R which can be expressed in terms of correlations over molecular quantities or, when evaluated by a hydrodynamic model, in terms of geometric factors. Both cases are discussed. Although R depends on the temporal behaviour of molecular quantities, i.e. it is a dynamic factor, it is actually a ratio of dynamic factors and can be approximated by static or equilibrium quantities. In the hydrodynamic limit, i.e. for brownian particles, this approximation is rigorously valid.

Theory of ultrasonically induced birefringence

An extended hydrodynamic theory of birefringence induced by ultrasonic waves in liquids composed of rigid symmetric top molecules is presented. This theory is closely related to that for HH depolarized light scattering spectra and to that of sound absorption. These phenomena involve the interactions of orientations with longitudinal or compressional modes, and the parameters that enter into the theory are the same as those involved in the corresponding theory of the interaction of orientations with shear modes, a theory which is applicable to flow birefringence and VH depolarized light scattering spectra. The theory predicts a linewidth expression for Brillouin scattering and a sound absorption coefficient for a liquid composed of anisometric molecules that differ from expressions previously derived; these expressions depend upon the frequency dependence of the rotational component of the shear viscosity but not upon the frequency dependence of the nonrotational component of the shear viscosity.

Light scattering studies of orientational pair correlations in liquids composed of anisometric molecules

The nature of the intermolecular forces which are important in determining the magnitude of orientational pair correlations (OPC) in liquids is studied by combining high pressure and depolarized Rayleigh light scattering techniques. For molecules of similar shape (toluene, bromobenzene, and nitrobenzene) we find that the magnitude of the OPC as measured by depolarized light scattering is not simply a function of the packing of the molecules. For 16 monosubstituted derivatives of benzene, we find that the magnitude of the OPC correlates well with the square of the molecular dipole moment. This correlation of OPC with dipole moment is understood by relating both the molecular dipole moment and the magnitude of the OPC to the detailed distribution of molecular electronic charge. The light scattering reorientational relaxation times for toluene, bromobenzene, and nitrobenzene as a function of pressure have also been measured and found to be linear functions of viscosity.

Dynamic orientational pair correlations in symmetric tops and static orientational pair correlations in anisometric molecules

The theory describing the depolarized spectrum of light scattered from symmetric tops is re-examined. On the basis of molecular symmetry it is shown that, in the rotational diffusion limit, dynamic orientational pair correlations do not affect the depolarized spectrum of scattered light. Molecular symmetry arguments are also applied to the calculation of the depolarized spectrum of light scattered from asymmetric tops and binary solutions of symmetric tops. It is found that orientational pair correlations may affect the calculated spectrum in these two cases in a rather complex fashion. Effects which had previously been attributed to the affect of dynamic orientational correlations may actually be explained on the basis of static orientational correlations alone. The experimental data on nitrobenzene and binary solutions of hexafluorobenzene/benzene are discussed explicitly. The possibility of obtaining information about the static orientational pair correlations of more than one axis in anisometric molecules is suggested.

Depolarized Rayleigh scattering in liquids: The density and temperature dependence of the orientational pair correlations in liquids composed of anisometric molecules

Structural analogs of the nematic liquid crystal p-methoxybenzylidene-n-butylaniline (MBBA) have been synthesized and the static orientational pair correlations (OPC) have been examined by light scattering. From the intensity of the depolarized Rayleigh scattered light, the OPC's have been determined in the neat isotropic liquid and in solution with carbon tetrachloride as a function of concentration and temperature for two of these molecules, p -methylbenzylidene-n -butylaniline (MBA) and p -ethylbenzylidene-n -butylaniline (EBA). The density dependence of the OPC's of these molecules and MBBA are analyzed in terms of the structure and polarity of the scattering molecules. The OPC's of the isotropic liquid trans -cinnamaldehyde have also been determined. We find that the molecular size and shape (repulsive forces) largely determine the extent of orientational order in these liquids. The attractive intermolecular forces affect the orientational order (static OPC) by increasing the density (at a given temperature). Several conclusions relating the static OPC and the onset of the nematic liquid crystalline phase are also discussed.

Molecular orientation and flow birefringence in simple liquids

The two different mechanisms which have been used to describe the orientation of anisometric molecules in a simple liquid subject to laminar flow lead to similar expressions for the angular-dependent distribution functions. The application of a static electric field to the flowing liquid is not capable of providing a test as to the validity of either mechanism, as was originally suggested by Tolstoi. A general optical treatment using the distribution functions relates the directions of the principal refractive indices and the birefringence with the magnitude of the applied hydrodynamic and/or electric field.